The present invention relates to procedures for melting iron-containing material, particularly sponge iron, in a steel-producing process.
In such processes, sponge iron produced by a direct reduction operation is usually melted in an electric arc furnace as a partial substitute for scrap and is then used in the production of steel. Since the grain size of such sponge iron is relatively uniform it can be easily delivered at a controlled rate and is therefore continuously fed into the arc furnace, during a melting period, through one or more openings in the cover of the furnace. The quantity of sponge iron thus fed in is here adapted to the amount of heat provided by the electrical energy being supplied so that all of the sponge iron melts immediately after introduction. In this type of feeding the melting and refining phases take place simultaneously.
In practice, sponge iron constitutes a limited proportion of the total material fed into the electric arc furnace. The furnace is sometimes also supplied with buckets of scrap, i.e. discontinuously. For this purpose electric arc furnaces in which sponge iron is processed are provided with a cover lifting and pivoting mechanism. Heat transfer by radiation which occurs predominantly in electric arc furnaces is advisable if buckets of bulky scrap protect the furnace walls from the radiation. As soon as all of the scrap has been made molten, i.e. during the transition from the melting to the refining phase, the walls are subjected to the greatest thermal stresses.
With continuous feeding in of sponge iron, the walls are subjected to continuous stresses as a result of the radiation heat so that they possess a relatively short lifetime. Moreover, the increased size of the layer of slag as a result of the use of sponge iron does not provide any protection against this radiation from the arc because the slag is ejected by the resulting eccentrically formed plasma stream. The furnace vessel will not be filled completely with all of the material to be melted. Whereas scrap is fed in discontinuously, sponge iron is supplied continuously in quantities per unit time which correspond to the amount of heat derived from the applied electrical energy. Consequently, the furnace contains almost exclusively molten slag and molten metal.
The drawback of this process is the discontinuous charging of the scrap which is necessary, however, inter alia, to protect the walls against excess heat stresses. With continuous melting of sponge iron such protection is not afforded so that the radiation heat subjects the walls to a high degree of wear.
Electric furnaces are known in which the electrodes are immersed into the melt. These furnaces, however, do not serve to melt grainy iron-containing material, particularly sponge iron, but serve to reduce ore mixtures in order to produce iron alloys, achievement of which involves a completely different mode of operation. The ore mixture, or burden, may consist, for example, of ores, slag formers and carbon carriers. The furnace is filled up to its rim with the burden so that a column of this burden is continuously present above the melt. Such a complete filling is necessary in order to achieve a maximum reduction output. The furnace wall in such a process is protected against excess heat stresses by the burden column.
The burden column which in this process is thus absolutely necessary is not subject to bridge formation due to the melting conditions of the burden employed. The physical properties of the bath and the chemical quality of the slag in this process are the inevitable result of the composition of the burden employed. Initially, the electrodes penetrate deeply into the reduction furnace and are completely surrounded by burden up to their upper edges. The object of this process is thus a high reduction output, which is the reason for the presence of the burden column above the melt, and penetration by the electrodes cannot be avoided.